1. Field of the Invention
The present invention relates to a piezoelectric transformer drive circuit that drives a piezoelectric transformer including a pair of primary electrodes and a secondary electrode, that steps up an AC voltage applied to the pair of primary electrodes and outputs the stepped up voltage from the secondary electrode. The present invention also relates to a cold cathode tube lighting apparatus including such a piezoelectric transformer drive circuit.
2. Description of the Related Art
Typically, a cold cathode tube is used as a backlighting light source of a liquid crystal panel. In a cold cathode tube lighting apparatus that lights the cold cathode tube, a piezoelectric transformer having a pair of primary electrodes and a secondary electrode is used to supply a high-voltage to the cold cathode tube. A known type of piezoelectric transformer drive circuit that drives such a piezoelectric transformer is a push-pull type transformer drive circuit as disclosed in, for example, Laid-open Japanese Patent Application No. 2004-39336. FIG. 4 shows a piezoelectric transformer drive circuit of the conventional push-pull type wherein feedback control is performed so as to maintain the AC voltage that is applied to the pair of primary electrodes of the piezoelectric transformer constant, and a cold cathode tube lighting apparatus including such a piezoelectric transformer drive circuit. The cold cathode tube lighting apparatus 101 includes a piezoelectric transformer drive circuit 105, a piezoelectric transformer 6 that is driven by the piezoelectric transformer drive circuit 105 and that outputs a high voltage from a secondary electrode, which high voltage is obtained by stepping up the AC voltage applied to a pair of primary electrodes A, B, a cold cathode tube 7 connected as a load to the secondary electrode of the piezoelectric transformer 6 and an impedance element 8, which is a resistor, connected in series with the cold cathode tube 7.
The piezoelectric transformer drive circuit 105 includes a tube current detection circuit (CDET) 111 that detects the signal of an impedance element 8, which is a signal that indicates the condition of a load that is connected to the secondary electrode, and that outputs the peak voltage or an averaged voltage thereof, a first error amplifier 112 that compares the output voltage of the tube current detection circuit 111 that is input at the inversion input terminal thereof and a first error reference voltage VREF1 that is input at the non-inversion input terminal thereof and amplifies and outputs the difference voltage, a voltage controlled oscillator (VCO) 113 that is controlled by the output voltage of the first error amplifier 112 and outputs an oscillation clock CLK of a reference period and a triangular wave signal TRI synchronized therewith, an applied voltage detection circuit (VDET) 115 that detects the AC voltage that is applied to a first primary electrode A of the pair of primary electrodes of the piezoelectric transformer 6 and is attenuated by an attenuator 114 including series-connected resistors, and that outputs the peak voltage or an averaged voltage thereof, a second error amplifier 116 that compares the output voltage of the applied voltage detection circuit 115 that is input at the inversion input terminal thereof and a second error reference voltage VREF2 that is input at the non-inversion input terminal thereof and amplifies and outputs the difference voltage, a PWM comparator 117 that compares the output voltage (the voltage at node C) of the second error amplifier 116 that is input at the non-inversion input terminal thereof and the triangular wave signal TRI of the voltage control oscillator 113 that is input at the inversion input terminal thereof and outputs a PWM signal P, a P-type MOS transistor 122 that inputs the PWM signal P of the PWM comparator 117 at its gate (node D) through an inversion buffer 119 and that has its source connected to an input power supply VCC, a frequency divider (DIV) 118 that divides in frequency the oscillation clock CLK of the voltage controlled oscillator 113, an N-type MOS transistor 125 that receives the output of the frequency divider 118 through a buffer 120 at its gate (node E), whose source is grounded, and whose drain is connected to the first primary electrode A of the piezoelectric transformer 6, an N-type MOS transistor 126 that receives the output of the divider 118 through an inversion buffer 121 at its gate, whose source is grounded, and whose drain is connected to a second primary electrode B of the pair of primary electrodes of the piezoelectric transformer 6, an inductor 123 one end of which is connected to the drain of the N-type MOS transistor 125 and the other end is connected to the drain of the P-type MOS transistor 122, an inductor 124 one end of which is connected to the drain of the N-type MOS transistor 126 and the other end is connected to the drain of the P-type MOS transistor 122, and a free wheeling diode 127 whose cathode is connected with the drain of the P-type MOS transistor 122 and whose anode is grounded.
Next, the overall operation of the cold cathode tube lighting apparatus 101 will be described. When an AC voltage is applied to the pair of primary electrodes A, B, the piezoelectric transformer 6 steps up the voltage by the piezoelectric effect and thus, outputs a high voltage from the secondary electrode. The cold cathode tube 7 is lit by application of this high voltage from the piezoelectric transformer 6. The step-up ratio of the piezoelectric transformer 6 depends on the frequency, as shown in FIG. 5 and has a peak at the resonant frequency f0. Accordingly, the power efficiency of the cold cathode tube lighting apparatus 101 has a peak practically in the vicinity of the resonant frequency f0. In the cold cathode tube lighting apparatus 101, the AC frequency at the primary electrodes A, B of the piezoelectric transformer 6 is therefore controlled by feedback of the tube current flowing in the cold cathode tube 7 such that the power efficiency is a maximum, by means of an impedance element 8 and piezoelectric transformer drive circuit 105.
Next, the operation of the piezoelectric transformer drive circuit 105 will be described. First of all, the voltage waveforms of the various sections for generating the AC voltage applied to the first primary electrode A of the piezoelectric transformer 6 are shown in FIGS. 6(a) to 6(f). FIG. 6(a) shows an oscillation clock CLK of reference period that is output from the voltage-controlled oscillator 113 and FIG. 6(b) is the triangular wave signal TRI thereof. FIG. 6(c) shows the PWM signal P that is output from the PWM comparator 117, which is output as a result of comparison of the triangular wave signal TRI and the output voltage (the voltage at node C) of the second error amplifier 116. The PWM signal P is output to the gate (node D) of the P-type MOS transistor 122 after inversion as in FIG. 6(d) by the inversion buffer 119. In contrast, the oscillation clock CLK is divided in frequency as indicated in FIG. 6(e) by the frequency divider 118 and is input to the gate (node E) of the N-type MOS transistor 125 through the buffer 120 and to the gate of the N-type MOS transistor 126 through the inversion buffer 121. These two transistors 125 and 126 are turned on and off alternately. When the N-type MOS transistor 125 is turned on and the P-type MOS transistor 122 is turned on, current flows from the input power supply VCC to the inductor 123, and energy is thus accumulated. In the next period, when the N-type MOS transistor 125 is turned off, voltage corresponding to the accumulated energy is generated as indicated in FIG. 6(f) and is applied to the first primary electrode A of the piezoelectric transformer 6. Also, although not shown, when the N-type MOS transistor 125 is turned off, the N-type MOS transistor 126 is turned on. When the P-type MOS transistor 122 is turned on, current flows from the input power supply VCC to the inductor 124 and its energy is accumulated. In the next period, when the N-type MOS transistor 126 is turned off, a voltage corresponding to the accumulated energy is generated and is applied to the second primary electrode B of the piezoelectric transformer 6.
Next, the operation for controlling the frequency of the AC voltage at the primary electrode A of the piezoelectric transformer 6 by feedback of the tube current flowing in the cold cathode tube 7 will be described. The tube current flowing in the cold cathode tube 7 is detected by the impedance element 8 and converted to a voltage signal. This voltage signal is detected by the tube current detection circuit 111 and the peak voltage or an averaged voltage thereof is output. The output voltage of the tube current detection circuit 111 and the first error reference voltage VREF1 are compared by the first error amplifier 112 and the difference between these two voltages is amplified and output. The voltage-controlled oscillator 113 is controlled by the output voltage of this first error amplifier 112 and an oscillation clock CLK of reference period corresponding to this voltage and a triangular wave signal TRI are output. As described above, the oscillation clock CLK is divided in frequency by the frequency divider 118 and is used to turn the two N-type MOS transistors 125 and 126 on or off alternately. So, the AC voltage is applied to the primary electrodes A, B of the piezoelectric transformer 6 with a period of twice the reference period. If, for example, the tube current flowing in the cold cathode tube 7 is more than a predetermined value, the frequency of the oscillation clock CLK of the voltage controlled oscillator 113 is increased, with a result that the AC frequency applied to the primary electrodes of the piezoelectric transformer 6 is also increased. On the other hand, if the tube current flowing in the cold cathode tube 7 is less than the predetermined value, the AC frequency applied to the primary electrodes of the piezoelectric transformer 6 is decreased. In this way, the tube current flowing in the cold cathode tube 7 is fed back, so that the AC frequency that is applied to the primary electrodes A, B of the piezoelectric transformer 6 is thereby controlled.
Next, the feedback control by which the AC voltage applied to the primary electrodes A, B of the piezoelectric transformer 6 is held constant will be described. The AC voltage that is applied to the first primary electrode A of the piezoelectric transformer 6 is attenuated by the attenuator 114 and is detected by the applied voltage detection circuit 115. The peak voltage or an averaged voltage thereof is output from the applied voltage detection circuit 115. The output voltage of the applied voltage detection circuit 115 is compared with the second error reference voltage VREF2 by the second error amplifier 116 and the difference between these two voltages is amplified and output. This output (node C) voltage, as described above, is compared with the triangular wave signal TRI of the voltage controlled oscillator 113 by the PWM comparator 117. The PWM signal P which is the result thereof is inverted by the inversion buffer 119 and output to the gate (node D) of the P-type MOS transistor 122. This output is used to control the time during which current flows from the input power supply VCC to the inductors 123 and 124, i.e., to control the energy accumulated in these inductors. For example, if the AC voltage applied to the primary electrodes A, B of the piezoelectric transformer 6 is larger than a predetermined voltage, the pulse width of the PWM signal P becomes small and the time during which current flows into the inductors 123 and 124 is shortened. On the other hand, if the AC voltage that is applied to the primary electrodes A, B of the piezoelectric transformer 6 is smaller than the predetermined value, the pulse width of the PWM signal P becomes larger and the time during which current flows into the inductors 123 and 124 becomes longer. Consequently, when the voltage of the input power supply VCC is high, the pulse width of the PWM signal P becomes small and the time during which current flows from the input power supply VCC to the inductors 123 and 124 becomes short. On the other hand, if the voltage of the input power supply VCC is low, the pulse width of the PWM signal P becomes large and the time during which current flows from the input power supply VCC to the inductors 123 and 124 becomes long. In this way, the effect of fluctuation of the input power supply VCC on the piezoelectric transformer drive circuit 105 is suppressed by feedback control such that the AC voltage that is applied to the primary electrodes A, B of the piezoelectric transformer 6 is kept constant. Hence, it is possible to prevent a decrease in power efficiency that results from drift in the detection of the tube current flowing in the cold cathode tube 7 as a consequence of fluctuation of the tube current caused by fluctuation of the input power supply VCC. For the input power supply VCC of a notebook personal computer, when a commercial AC power source is used and when a battery is used, there is a fluctuation in a voltage range of, for example, about 9 V to about 21 V. So, the cold cathode tube lighting apparatus 101 is particularly advantageous in circumstances where it is used in a notebook personal computer.
Recently, however, a piezoelectric transformer drive circuit of a full-bridge type (see, for example, Laid-Open Japanese Patent Application No. 2001-136749) has been developed in which both primary electrodes of a piezoelectric transformer are driven by a power supply side transistor and a ground side transistor in order to further improve the power efficiency beyond that of a piezoelectric transformer drive circuit of push-pull type. It is said that such a piezoelectric transformer drive circuit of a full bridge type makes it possible to achieve at least 90% power efficiency, compared with about 80% power efficiency for a push-pull type circuit. The inventor of the present application theorized that ,if feedback control, described above, of the AC voltage that is applied to the primary electrodes of the piezoelectric transformer is refined and is applied to such a piezoelectric transformer drive circuit of a full bridge type, the effect of voltage fluctuation of the input power supply VCC could be minimized and it might thereby be possible to realize a piezoelectric transformer drive circuit having high efficiency in a correspondingly wider voltage range of the input power supply VCC.